On 13 March 2008, the International Space Station (ISS) passed across the field-of-view of Germany's remote sensing satellite, TerraSAR-X, at a distance of 195 kilometres (122 miles) and at a relative speed of 34,540 kilometres per hour (or over 22,000 mph).
In contrast to optical cameras, radar does not ‘see’ surfaces. Instead, it is much more aware of the edges and corners which bounce back the microwave signal it transmits. Smooth surfaces such as those on the ISS solar generators or the radiator panels used to dissipate excess heat, unless directly facing the radar antenna, tend to deflect rather than reflect the radar beam, causing these features to appear on the radar image as dark areas. The radar image of the ISS therefore looks like a dense collection of bright spots from which the outlines of the space station can be clearly identified. The central element on the ISS, to which all the modules are docked, has a grid structure that presents a multiplicity of reflecting surfaces to the radar beam, making it readily identifiable. This image has a resolution of about one metre. In other words, objects can be depicted as discrete units – that is, shown separately – provided that they are at least one metre apart. If they are closer together than that, they tend to merge into a single block on a radar image.